Ok, so here is all of the technical, yet important stuff you will need to know so that you will never get ripped off on a brake job again.
Once the test drive has been performed the technician will lift the vehicle and remove the wheels. A good technician will always be attentive to any clues that may help him solve the problem. On front brakes (which are all disc and pad type, except for old cars) the rotors (discs), pads (the friction material), and the caliper (the hydraulic piston assembly that pushes the pads) are all clearly visible. On rear brakes (many of which are drum and shoe type) the drum will be removed to view the shoes and brake hardware. These drums are normally on fairly tight, so don’t be surprised to see a hammer taken to it to loosen it.
Now, let me walk you through what we are looking for when we look at brakes. First of all we will look at the thickness and condition of the brake pads and shoes. We normally recommend replacing friction material when it gets to 2/32″ (This is the same recommendation made by the Motorist Assurance Program (MAP), an industry recognized program that provides standards for the auto repair industry, and guidelines to give consumers information they need to make educated decisions about repairing their vehicle.). There are, however, several other conditions that may require the pads or shoes to be replaced. Cracked pads obviously need to be replaced. Even hairline cracks indicate overheating, which diminishes the effectiveness of the pad. Glazing is another indication of overheating and is another condition in which the pad should be replaced. A glazed pad will look really shiny because it has hardened. Grooving or scarring will also call for pad replacement since there is an uneven contact surface. Brake noise (squealing) from an otherwise good pad may just be produced simply by a low quality pad. We will only replace these pads if the client insists.
The rotor is the disc that the pad makes contact with to stop the vehicle (there is a pad on both sides of the rotor). The drum does the same job except that it is round with curved shoes on the inside. Similar problems are looked for here. Any cracking, pitting, or glazing indicates excessive heat. Replacement is recommended, although you might be able to get away with resurfacing a glazed rotor or drum. Rust is a major problem that may prevent any resurfacing from being done at all.
Rotors and drums should be carefully measured using micrometers. In many cases, these parts have the minimum acceptable thicknesses forged onto them. Otherwise, the technician should refer to the manufacturer’s specifications (either in a catalog or labor guide) which will tell him how much metal can be cut when resurfacing, and when the part should be discarded. Your technician should always present these numbers to you if they are recommending replacement. If you doubt that you are being told the truth, have the technician measure the rotor or drum in front of you (tip: make sure they zero out the micrometer before taking the reading). Each cut on a lathe, by the way, will remove about 3/1000″ (or .003). A badly grooved rotor, for example, may require several cuts to provide a smooth surface. If it is real bad, a smooth surface may not be achievable without cutting beyond the specifications.
One more problem with rotors and drums is warping (the technical term on a rotor is runout). Runout on front rotors can cause your front end to shake when you brake (side to side, or forwards and backwards), or you may feel your brake pedal pulsate up and down, especially at slower speeds. Warped rotors are caused by two things, generally: 1) Excessive heat can be generated from a rotor that has gotten too thin, or maybe even by riding the brakes. The heat can also increase when towing, or driving down steep inclines. 2) Over-torquing lugnuts will cause rotors to warp. Many shops do not use torque sticks to limit how tight lugnuts are installed. This is a pretty common problem overlooked by many mechanics.
Runout can be measured with a micrometer that mounts to a shock or body panel. If the runout (warp) is more than 3/1000″ (.003) it is normally recommended to replace the rotor. Sometimes, it is possible to cut the warp out of the rotor on a lathe, and we normally opt for this to save our clients money. A drum’s warp can be measured as well, but is more clearly visible by the wear pattern on the inside of the drum. A warped drum will normally show wear from the shoes on the short axis, but no wear on the long axis. Another problem that drums may suffer from is bell wear. This means that the diameter of the open end is smaller than the closed end. This can be seen if there is difficulty in reinstalling the drum over the shoes. Once on the shoes, however, the drum will spin freely, indicating that there is more space and no contact with the shoes. If there is enough thickness to the drum, this can be cut out on a lathe as well.
[A quick word about resurfacing. Some mechanics will tell you that resurfacing is not necessary if the surface of the rotor or drum looks smooth. However, that rotor, for example, is married to its pad, creating imperfections and fine grooving. Placing a new pad on an unresurfaced rotor will result in uneven contact and faster wear on the new pad. It is possible that you will experience brake noise as well. Our opinion is that rotors and drums should be resurfaced every time pads and shoes are replaced.]
Brake hardware is the third component completing this trifecta. On disc brakes, hardware is made up of clips and shims that hold the pad in place. Hardware doesn’t always need to be replaced. If it does not fit snugly in the caliper bracket, nor hold the pad firmly in place, it will move around and cause brake noise. On drum brakes, hardware is made up of several springs, plates, brackets, and adjusters. Again, it really only needs to be replaced if it is not doing its job anymore. If its rusty, count on it being replaced. Otherwise, don’t discount a good “clean and adjust.” The technician will thoroughly clean all brake dust, debris, and contaminants, lubricate the hardware, and adjust the hardware to ensure the parking brake works properly (tip: Can’t get an inspection sticker because your parking brake won’t hold? You may just need a clean and adjust.).
Most of the rip offs in the brake industry happen right here. A shop may sell pads or shoes unnecessarily, or will try to sell you rotors or drums even though they can still be resurfaced. Read on to discover more about the brake inspection that many shops don’t even look at.
4 responses so far ↓
V. Hom // September 20, 2008 at 1:34 am |
1. If a pulsating noise is elicited when braking at lower speeds (not in highway) what are the possible causes?
2. In what instances will the brake caliper be dislodged completely, and can a potential problem with the calipers be noted on visual inspection?
3. How does one test for problems with the bearing and hub?
Thank you.
ieat // September 28, 2008 at 1:45 am |
Good questions, and sorry for the late response.
1. This noise could be a warped rotor. Other things you may experience with a warped rotor are a shaking in the front end of the vehicle upon braking and maybe even a pulsating pedal at lower speeds. The vehicle will seem to pulse forward and backward when braking at lower speeds.
2. A caliper can come loose if the bolts holding it in place are missing, or if they break. It is very difficult to visually see potential problems with calipers. The main problem readily visible is brake fluid leaking from the caliper. My shop recommends replacing calipers as a preventative measure at 90k. Some shops even earlier. Vehicles in northern states where salt and snow are prevalent suffer from faster degredation of brake components.
3. A bad bearing may sound like a loud hum coming from the wheel. The sound may even disappear when turning one way or the other. Any play in the bearing dictates that it should be replaced. You will need to raise the wheel off the ground, grab the tire at 12 and 6 o’clock, and try to push and pull the wheel. A bad bearing will feel like the wheel is loose. In fact, it may only move very slightly (1/8″ even). I’ve heard some bearing make a grinding sound because the bearings have crushed. Now, some vehicles have a bearing pressed into the hub, which would require replacing the assembly (somewhat expensive). A hub, just like a rotor, can have a tendency to warp. The only way to check this, though, is to measure the warp (called runout) with a dial indicator. This is a tool that can measure warp in thousandths of an inch.
Hope this helps.
DJ Murray // May 24, 2009 at 2:30 am |
Ok, after reading this page, I am still a little confuzed on what my problem may be.
I have a pulsating thump coming from the rear passenger side of my car. The easiest way to explain it is it sounds similar to a bicycle with a baseball card in the spokes. The faster I am going when I apply the brakes, the faster the pulsating, then as I am coming to a complete stop, the pulsating is spread out and I can hear each thump like someone knocking on the car.
I have taken my car to a few different mechanics, and neither one of them can figure it out. It may be because the problem lies beneath, and all they look at is the outside. Neither one has taken the wheel off to expose the brake system.
Another issue I am having: I ride the brakes when approaching a stop light, my front brakes seem to lock up and make a loud hum. I let off the brake and it’ll stop and go back to normal.
I drive a 2002 Chevy Malibu. Approx 135,xxx miles. I bought the car used, and have no knowledge of any previous brake jobs on it.
ellen // June 29, 2009 at 4:01 pm |
hydrolic brakes slide stop pedal does down after stop?